The present invention relates to a process of preparing styrene oxide by the reaction of styrene and hydrogen peroxide in the presence of a catalyst.
Styrene oxide is used over a wide range of field, for example as a stabilizer for polymers, an ultraviolet ray absorber, a starting material in the preparation of drugs, a stabilizer for solvents, or as a starting material for phenethyl alcohol and phenethyl aldehyde which are useful as synthetic perfumes and sweetening materials.
For preparing styrene oxide by the epoxidation of styrene there generally is adopted a process in which styrene is epoxidized using an organic peracid, as described in Japanese Patent Laid Open No. 149271/1980. However, this process involves the following drawbacks and is not always satisfactory.
(1) During the reaction of oxidizing styrene with an organic peracid, the organic peracid is decomposed and there occurs an addition reaction of the resulting radical to styrene, thus resulting in that the selectivity of styrene oxide with respect to styrene is deteriorated. PA1 (2) The resulting styrene oxide cleaves in the presence of an organic acid byproduced after the reaction, thereby producing an ester and a hydroxy compound, whereby the selectivity of styrene oxide with respect to styrene is deteriorated. PA1 (3) Peracetic acid which is most easily available industrially among organic peracids is prepared by a so-called Daicel-Wacker process comprising air oxidation of acetaldehyde, but it is a very expensive oxidizing agent. PA1 (4) In order to avoid a possible danger in the use of an organic peracid it is necessary to pay close attention to both operation and equipment.
On the other hand, an oxidation reaction using hydrogen peroxide byproduces only water and does not cause the problem of environmental pollution; besides, hydrogen peroxide is easily available industrially and is inexpensive. In principle, therefore, hydrogen peroxide is a desirable epoxidizing agent. In the preparation of an epoxide by the reaction of styrene and hydrogen peroxide, however, the styrene conversion and the selectivity to the epoxide are both low. The low conversion is because hydrogen peroxide remains unreacted in the reaction performed at a low temperature, while in the reaction performed at a high temperature hydrogen peroxide decomposes and produces oxygen, and thus hydrogen peroxide is not effectively consumed in reaction.
The reason why the selectivity to epoxide is low is that water which is introduced into the reaction system together with hydrogen peroxide and water resulting from the reaction both cause the formation of polyol.
The reactivity of styrene in epoxidation is as tabulated below (see "Encyclopedia of Polymer Science and Technology" Vol. VI (1967), Interscience Publishers, New York, p. 83). The epoxidizing speed of styrene is relatively low in comparison with other olefins; for example, it is about one tenth as compared with a relative reactivity in the epoxidation of cyclohexene, thus indicating that the epoxidation reaction rate of styrene is very low.
______________________________________ Olefin Relative Reactivity ______________________________________ CH.sub.2 .dbd.CH.sub.2 1 C.sub.6 H.sub.5 CH.sub.2 --CH.dbd.CH.sub.2 11 R--CH.dbd.CH.sub.2 25 Ar--CH.dbd.CH--Ar 27 Ar--CH.dbd.CH.sub.2 60 Ar--CH.dbd.CH--R 240 (Ar).sub.2 C.dbd.CH.sub.2 250 R--CH.dbd.CH--R 500 (R).sub.2 C.dbd.CH.sub.2 500 Cyclohexene 675 Cycloheptene 900 Cyclopentene 1000 (R).sub.2 C.dbd.CH--R 6500 (R).sub.2 C.dbd.CH(R).sub.2 &gt;&gt;6500 ______________________________________
In the above table, Ar and R represent aryl and alkyl, respectively.
In order to solve the above-mentioned problems involved in the preparation of styrene oxide by the reaction of styrene and hydrogen peroxide, there has heretofore been proposed the use of a specific catalyst.
For example, according to J. Org. Chem., 53, 1553, (1988), styrene oxide is obtained in 74% yield (based on hydrogen peroxide) if a quaternary ammonium salt of phosphotungstic acid is used as a hydrogen peroxide epoxidizing catalyst. Although this reported process is greatly improved in the yield of styrene oxide as compared with other conventional processes, it is difficult to adopt it on an industrial scale because the quaternary ammonium salt (an interphase transfer catalyst) used as a catalyst component is every expensive.
In Japanese Patent Laid Open No. 129276/1980 there is proposed a process wherein styrene and hydrogen peroxide are reacted in the presence of arsenic oxide and 3,5-di-tertbutyl-4-hydroxytoluene. However, a combined use of arsenic oxide with aqueous hydrogen peroxide involves such drawbacks as rapid decomposition of hydrogen peroxide and a low epoxidizing speed. Further, since arsenic compounds are strong in toxicity, it is necessary to pay close attention to the manufacturing equipment to prevent poisoning during production and also during use of the resulting products with the arsenic compounds incorporated therein.
In U.S. Pat. No. 3,806,467 there is proposed a process wherein an olefin and hydrogen peroxide are reacted in the presence of a bis(tri-n-methyltinoxy)molybdic acid catalyst to prepare an epoxide. However, as long as the working Examples thereof are reviewed, the yield of styrene oxide is a little lower than 3% (based on hydrogen peroxide) and thus this proposed process cannot be considered preferable as a styrene oxide preparing process although the yield of cyclohexene epoxide is high and the process in question is effective as a cyclohexene epoxide preparing process. It is presumed that the low yield of styrene oxide in the said process is because the resulting styrene oxide cleaves oxidatively and byproduces benzaldehyde and further benzoic acid.
The bis(tri-n-methyltinoxy)molybdic acid catalyst described in the above U.S. Pat. No. 3,806,467 is inexpensive and easily available industrially and can be fixed to active carbon and also to organic adsorbent resins, thus permitting the reaction to be carried out in a heterogeneous catalyst system and thereby permitting easy separation of the catalyst from the reaction system.
It is the object of the present invention to solve the above-mentioned problems of the process proposed in the foregoing U.S. Pat. No. 3,806,467 and provide an improved process capable of suppressing the formation of byproducts and affording styrene oxide in high yield under application of the process proposed in the said U.S. patent to the preparation of styrene oxide from styrene.